Conventionally, a variable transmission ratio device which uses a differential mechanism and adds rotation based on motor driving to rotation of an input shaft based on a steering operation and transmits the added rotations to an output shaft is known. Patent Document 1 discloses a housing-fixed type variable transmission ratio device in which a housing for housing a differential mechanism and a motor is fixed to a vehicle body of a motor vehicle. The variable transmission ratio device disclosed in this document includes a locking device. The locking device suppresses idling of a motor shaft when power supply to the motor is stopped, and restrains rotation of the motor shaft to enable torque transmission between the input shaft and the output shaft.
As shown in FIG. 9, a locking device 81 described in Patent Document 1 includes a lock holder 83 integrated with a motor shaft 82, and a lock arm 84. The lock arm 84 is provided on a housing (not illustrated) fixed to a vehicle body. On an outer peripheral surface 83a of the lock holder 83, engagement grooves 85 are formed. Each of the engagement grooves 85 has end wall portions 86 on both end portions in the circumferential direction. When the lock arm 84 is inserted into the engagement groove 85 and an engagement portion 87 of the lock arm 84 is engaged with one of both end wall portions 86, rotation of the motor shaft 82 is locked. When the lock holder 83 rotates in a state where a side surface 86a of the end wall portion 86 is engaged with the side surface 87a of the engagement portion 87, the engagement portion 87 is pulled radially inward by the side surface 86a. Therefore, in the locked state, even when a torque is input into the lock holder 83, the lock arm 84 does not come out from the engagement groove 85. The locking device 81 includes a tolerance ring 91 between the motor shaft 82 and the lock holder 83. In FIG. 9, for convenience of description, the tolerance ring 91 is illustrated in an exaggerated manner.
The tolerance ring 91 includes a ring main body 92 formed by curving a band-shaped metal plate substantially into a C-shape. On the ring main body 92, spring-like portions 93 elastically deformable in the radial direction are formed. In the case of Patent Document 1, the spring-like portions 93 consist of a plurality of protrusions 94 protruding radially outward from the vicinity of the center of the ring main body 92. The protruding amounts of the protrusions 94 are fixed. The protrusions 94 are respectively disposed at even angular intervals in the circumferential direction. The tolerance ring 91 is press-fitted between the motor shaft 82 and the lock holder 83 in a state where the respective protrusions 94 are radially compressed. Therefore, on the tolerance ring 91, frictional resistance is caused by the elastic forces of the respective protrusions 94. Accordingly, the frictional resistance between the spring-like portions 93 of the tolerance ring 91 and the lock holder 83 restricts the motor shaft 82 and the lock holder 83 from rotating relative to each other. When a torque equal to or more than a predetermined value is input, the spring-like portions 93 of the tolerance ring 91 become slipping surfaces and allow the motor shaft 82 and the lock holder 83 to rotate relative to each other. Thus, the tolerance ring 91 functions as a torque limiter. By allowing the motor shaft 82 and the lock holder 83 to rotate relative to each other by the tolerance ring 91, when the differential mechanism is abnormal, a steering operation can be continuously performed. A maximum torque at which the motor shaft 82 and the lock holder 83 can be restricted from rotating relative to each other is referred to as a slipping torque.
Relative rotation of the motor shaft 82 and the lock holder 83 is allowed by the tolerance ring 91 for a fail-safe measure that enables a steering operation when the differential mechanism is abnormal. Therefore, relative rotation of the motor shaft 82 and the lock holder 83 in a normal state is not preferable from the viewpoint of security of reliability, etc., of the device. However, in a case where steering to a steering end is performed and then further steering over the steering end is performed, when the motor shaft 82 is locked during high-speed rotation, at the moment at which the engagement portion 87 of the lock arm 84 is engaged with the end wall portion 86 of the engagement groove 85, a great impact is applied to the lock holder 83.
In detail, when the end wall portion 86 collides with the engagement portion 87 at the time of locking, the lock arm 84 is pulled radially inward by the side surface 86a of the end wall portion 86. Accordingly, the engagement portion 87 also collides with the bottom surface of the engagement groove 85. When an impact is thus applied to the lock holder 83 in both the circumferential direction and the radial direction, the shaft center of the lock holder 83 may deviate from the shaft center of the motor shaft 82. At this time, while the compressed amount of the protrusions 94 near the lock arm 84 increases, the compressed amount of the protrusions 94 on the opposite side of the lock arm 84 decreases.
Here, when the protrusion 94 is compressed to near the limit of the elastically deformable range, the increased amount of the elastic force according to the increase in the compressed amount becomes smaller. In particular, the tolerance ring 91 to be used in the variable transmission ratio device is small in size, and the protruding amounts of the protrusions 94 are also as small as 1 mm. Therefore, the protrusions 94 are easily compressed to near the limit of the elastically deformable range. Therefore, the increased amount of the elastic force of the protrusion 94 the compressed amount of which was increased becomes relatively small, and the decreased amount of the elastic force of the protrusion 94 the compressed amount of which was decreased becomes relatively large. As a result, the frictional resistance of the whole tolerance ring 91 becomes small, and the slipping torque decreases at the time of locking. Accordingly, the motor shaft 82 and the lock holder 83 are likely to rotate relative to each other.
Therefore, to secure a sufficient slipping torque at the time of locking, it can also be considered that the protruding amounts of the protrusions 94 are increased and the slipping torque of the tolerance ring 91 is set to be large in advance. However, in this case, in a state where the shaft center of the motor shaft 82 and the shaft center of the lock holder 83 match each other, the slipping torque becomes excessively large. The above-described problem occurs not only in a variable transmission ratio device but also in a torque limiter including a shaft-shaped first rotating member, a second rotating member that is fitted on the first rotating member, and a tolerance ring disposed between the first and second rotating members when an impact is applied to the second rotating member.